1. Field of the Invention
The invention relates to staring IR detector arrays in which two two-dimensional images of an object field are simultaneously readout, and more particularly to one in which medium wave IR (MWIR) and long wave IR (LWIR) detector area arrays are registered, pixel for pixel, for image coincidence.
2. Prior Art
A staring two dimensional focal plane configuration for IR detection that is capable of simultaneous, superimposed, dual band MWIR and LWIR pixel-registered, image readout is known. The advantage of a dual band focal plane array (FPA) of this kind is that it leads to a compact design for the IR sensor requiring only a single optical image input and not requiring optical beam splitters, electrical or optical switches, or other means to superimpose the images upon spatially separated arrays. The dual band design ideally provides continuous, staring, simultaneous, and precisely registered image-sensing in both spectral bands.
Available technology-has provided such compact dual band, focal planes with scanned linear detector arrays, but not with staring two-dimensional (area) arrays. With the simpler scanned linear arrays it is possible to superimpose a MWIR detector array with its associated readout multiplexing circuitry or "MUX" over a LWIR detector array and its "MUX", with the detector arrays in the focused optical beam and the optically opaque multiplexers displaced to one side of the focused optical beam.
The widest dynamic range, most sensitive IR detector arrays are photovoltaic (PV) diodes, which require a one-to-one connection between each detector pixel and a preamplifier dedicated to each pixel as a necessary part of the MUX. Since the multiplexer integrated circuit chips are not IR transmissive due to diffusion wells, circuit runs and other active silicon CMOS circuitry, it is not possible to transfer images through the PV diode MUX assuming pixels arranged in two dimensions at customary densities. Thus, in the known configurations for a dual band linear focal plane array the IR opaque MWIR-MUX must be translated out of the optical image beam as it passes to an underlying linear LWIR array and MUX.
When a dual band focal plane array is a staring area detector array, the artifice of translating the MWIR-MUX out of the way of an optical input to a single line of pixels, customary in line arrays, will not work. Directly overlaying a LWIR area detector array and MUX, with a PV MWIR area detector array and MUX, to form a dual band, staring, area detector array will fail because the PV MWIR area array and the MUX may be expected to block the LWIR optical input to the LWIR area detector array.
In another development, new detector device research has been directed to development of monolithic, time-switched photovoltaic diode (PV) structures. In this case the staring detector array is electrically switched to respond to one of the two (MWIR or LWIR) spectral bands. This approach lacks the desirable feature of providing simultaneous response to both bands.
In copending application Ser. No. 07/832,799 of Milton Noble, a focal plane for dual band IR sensing of a two dimensional image is described, the arrangement providing coincident simultaneous images. In that arrangement, a four chip arrangement is described in which a mid wave array indium bumped to its MUX, is mechanically aligned to a LWIR array and MUX with a small gap. The first chip is a MWIR MUX with an apertured layout having transparent windows, permitting IR illumination of the pixels of an underlying MWIR detector array formed on a second chip. The MWIR detector array is of the Metal Insulator Semiconductor (MIS) Charge Injection Device (CID) variety, in which the pixels can be transparent to lower energy long wave IR. The third chip is a backside illuminated LWIR PV diode array, which is connected to a fourth chip containing an opaque LWIR MUX on the optical axis but beyond all optical paths, which are suitable locations for the PV diode detector array and the LWIR MUX.